# Patent

## Title
Optical device using laterally-shiftable diffraction gratings

## Abstract

A double-sided grating divider acts as a light switch where the upper and lower grating dividers are arranged to accommodate a relative lateral shift therebetween of about one-fourth of the period of the diffraction grating elements and where the critical refraction angles of the grating dividers are more than about 43.6°. Lateral shift may be achieved by various devices including MEMS and metal couplers having a known/calibrated thermal coefficient of expansion over a temperature range of interest.

## Background
FIELD OF THE INVENTION
This invention relates to diffraction grating dividers and more particularly to an optical device comprising a double-sided grating divider made up of mutually inverted optical grating elements which can be shifted laterally relative to one another to modulate the degree to which light is transmitted therethrough.
BACKGROUND OF THE INVENTION
It is known that diffraction-based grating dividers can be constructed using materials of different diffraction indices and certain critical geometries. For example, a grating divider may comprise a periodic pattern of geometrically regular ridges and grooves in a substrate of fused SiO2. The ridges and grooves exhibit geometric characteristics including a period “p”, a height “h” of the ridges, and a fill factor “r” which is the ratio of the width of the ridges to the period. Through selection of these parameters, it is possible to determine the degree to which light of various orders are transmitted and/or are trapped within the substrate. For example, a grating having a refraction angle in excess of the 43.6° critical angle for the SiO2 air interface will trap±first order refraction component light. Details of a relevant grating divider can be found in the co-pending application Ser. No. 12/638,334 filed Dec. 15, 2009 and assigned to the assignees of this application, the entire content of which is incorporated herein by reference.
SUMMARY OF THE INVENTION
The present invention provides a variable transmissivity optical device in the form of a double-sided grating divider made up of mutually inverted first and second grating dividers in which transmissivity of normal incident light of the 0th order is decoupled; i.e., goes to near 0, when the mutually inverted grating dividers are shifted relative to one another by approximately one-quarter of the period of the grating elements whereas the same 0th order normal incident light is at least 95% transmitted when the grating elements in the dividers are aligned. When the diffraction angles of the grating dividers exceed the critical angle, all higher order components are decoupled.
The phenomenon of the present invention can be used to practical advantage in a number of ways including the modulation or switching of laser light as well as the control of the transmissivity of the unpolarized sunlight through, for example, a window or skylight.
In one preferred form hereinafter described in detail, the grating dividers comprise geometrically regularly distributed grating elements of a first material such as TiO2 immersed in air and mounted on a substrate of a second material such as fused SiO2 wherein the geometries; i.e., the p, h and r values, of the grating dividers are such as to produce a diffraction angle of greater than about 43.6°.
When arranged in a mutually inverted spaced-apart relationship in such a way as to allow a lateral shift between two grating dividers, the degree to which normal incident light in the visible range is transmitted through the grating dividers can be switched from less than about 5% to more than about 95%. The invention, therefore, can be implemented in various ways to act as a light valve or regulator. The immersion substance need not be air but can be solid, liquid or gas substances the refraction indices of which with the TiO2 interface are known.
As hereinafter discussed in detail, the variation in alignment may be singular; i.e., one of the grating dividers may be fixed while the other is permitted to shift laterally. Alternatively, it may be mutual or bilateral in that both grating dividers are arranged and/or mounted so as to be laterally shiftable, but in opposite directions.
The mechanisms for providing the lateral shift may vary considerably. In one practical arrangement the grating dividers or multiples thereof may be mounted strategically on structures which carry other structures or patterns, the degree of alignment between which is critical. In another arrangement, the shifting mechanisms may be in the form of piezoelectric devices or microelectromechanical systems (MEMS). In a still further embodiment, the shift producing elements may be devices with predetermined and precisely known coefficients of thermal expansion such that the degree of lateral alignment between the grating dividers and the consequential degree of visible light transmissivity therethrough varies as a function of temperature.
The invention and the various embodiments and applications thereof may be best understood from a reading of the following specification which is to be taken with the accompanying drawings.



BRIEF SUMMARY OF THE DRAWINGS
The description herein makes reference to the accompanying drawings wherein like reference numerals refer to like parts throughout the several views and wherein:
FIG. 1 is a diagram of a double-sided diffraction grating divider employing the principles of the present invention with the grating elements of the dividers in lateral alignment so as to fully couple the 0th order incident light component therethrough;
FIG. 2 is a graph of light wavelength vs. transmittance for the device of FIG. 1 in the aligned condition illustrated in FIG. 1;
FIG. 3 is a diagrammatic illustration of a double-sided grating divider but with the grating elements shifted by a distance d;
FIG. 4 is a graph of wavelength vs. transmittance for the double-sided grating divider of FIG. 3 in the shifted or non-aligned condition;
FIG. 5 is a perspective view of an alignment marker for a multi-layer fabrication process utilizing the principles and physical implementations of the present invention;
FIG. 6 is a cross-sectional view of an optical switch using MEMS to shift the upper diffraction grating divider of the two inverted diffraction grating dividers relative to the lower diffraction grating divider wherein a liquid crystal layer serves as a fluid interface between the two diffraction grating dividers; and
FIG. 7 is a diagrammatic illustration of a second application of the present invention in a switch for unpolarized sunlight in which the lateral movement or shift of the grating dividers in the double-sided grating assembly is accomplished by means of metals with known and calibrated coefficients of thermal expansion.



DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS
Referring to FIG. 1, there is shown a double-sided device 10 comprising mutually inverted and laterally aligned diffraction grating dividers 12, 14. Grating divider 12 comprises a substrate 16 of fused SiO2 on which identical diffraction grating elements 18, 20, 22 made of TiO2 are fused or mounted to the top surface of the substrate 16 in a regular periodic fashion so as to exhibit a period p of 540 nm, a height h of 175 nm and a width of 145 nm. The elements 18, 20, 22 are immersed in this case in air to create an air/TiO2 interface. Other surrounding media, including solid substances, can also be used. The fill factor r=0.27 can be determined by dividing the width w by the period p. The geometry is selected so as to produce a refraction angle in excess of the 43.6° critical angle of the TiO2 air interface. In a practical embodiment, the period p may be in the range of about 0.87 to about 0.93λ, where λ is the wavelength of the incident light 24, the fill factor r is in the range of about 0.22 to about 0.32 and h is in the range of about 0.24 to 0.34λ. As shown in FIG. 1, with these values, the first order refraction components are diffracted by an angle of approximately 50°, well above the critical angle.
The lower diffraction grating divider 14 comprises a substrate 26 of SiO2 and periodically arranged TiO2 grating elements 28, 30, 32 also immersed in air and having the exact same geometry as the grating elements 18, 20, 22. In other words, grating divider 14 is identical to grating divider 12 but is inverted. In the example of FIG. 1, light 24 may be thought as the input or incident light whereas light 34 is the output light which is transmitted through; i.e., the output light component when the transmittance or transmissivity 10 is operating at the “on” level hereinafter described.
FIG. 2 is a graphical illustration of the degree of the transmittance of the 0th order of the s-polarized incident light 24 when the diffraction grating elements 18, 20, 22 of the upper diffraction grating divider 12 are fully laterally aligned with the grating elements 28, 30, 32 of the lower diffraction grating divider 14. The s-polarized light has the electric field in the y axis. Between about 550 nm and about 670 nm; i.e., in the “on” zone 36, the transmittance is in excess of 95%.
Looking now to FIG. 3, the double-sided grating divider device 10 is shown in the “off” condition wherein the lower grating elements 28, 30, 32 are shifted by a distance d relative to the upper grating elements 18, 20, 22 wherein d is approximately p/4. FIG. 4 shows that the transmittance of the 0th order component of normal incident unpolarized light in the wavelength range between about 550 and 650 nm is in the “off” zone 38 wherein the transmittance is near 0; i.e., less than about 5%.
Referring to FIG. 5, there is shown a first practical application of the phenomenon of the present invention in an alignment marker for a SiO2 multi-layer fabrication process. This device comprises an upper planar substrate 40 and, in spaced parallel relationship therewith a lower SiO2 substrate 42. The upper substrate 40 carries TiO2 grating elements 44, 46, 48, 50 in the four corners thereof arranged essentially in the fashion illustrated in FIGS. 1 and 3. The lower SiO2 substrate 42 carries diffraction grating elements 52, 54, 56 and a fourth diffraction grating divider in the upper left hand corner as viewed in FIG. 5 which is obscured by the drawing. The diffraction grating elements 48, 52, for example, are arranged to be essentially in alignment with one another when the substrates 40, 42 are properly aligned but will be shifted relative to one another when the substrates are non-aligned along a lateral axis passing through and between the substrates 40, 42. A similar alignment situation exists for the grating elements 50, 56 and for the grating elements 46, 54 as well as the grating elements in the upper left hand corners which only the upper grating element 44 is visible.
Light 58 from a laser is directed normally toward the upper substrate 40 in such a way that when the grating elements are laterally aligned so as to exhibit the on condition represented by FIG. 2, the 0th order incident light component passes or is transmitted substantially fully through to a detector 60 to show the aligned condition. Conversely, when the substrates 40, 42 are not aligned, indicating an alignment error in the fabrication process, substantially no light from the laser 58 reaches the detector and the fabrication process is thereafter terminated or paused to make suitable alignment adjustments. Similar alignment detection can be achieved for the orthogonal axis.
Referring now to FIG. 6, there is shown an optical device in the form of an incident light valve or switch 62 comprising the combination of a planar upper SiO2 substrate 64 carrying periodically arranged TiO2 diffraction grating elements 66 and, in spaced parallel relationship thereto, a lower planar SiO2 substrate with TiO2 diffraction grating elements 72. The lower substrate 70 is fixed in a frame 68 whereas the upper substrate 64 is sandwiched between microelectromechanical systems (MEMS) devices 74, 76 so that the substrate 64 can be shifted relative to the lower substrate 70 to align and/or shift the diffraction grating elements 66, 72 to act as a valve whereby the incident light 78 is switched between a low transmissivity condition wherein the output light 80 is essentially 0 and a high transmissivity wherein the output light component 80 is nearly 100%. The lateral shifting of the substrates 64, 70 is facilitated by means of a liquid crystal layer 82 which lies in the space between the inverted substrates 64, 70 and has a refraction index of approximately 1.6. This layer can be replaced by an air layer with a refractive index of 1.
Referring to FIG. 7, there is shown a temperature sensitive skylight 84 for a room 86. The skylight 84 comprises an upper planar SiO2 substrate 88 having periodically spaced diffraction grating elements 90 mounted to the upper surface thereof and exposed to incident sunlight 104. The thermally responsive skylight 84 further comprises a lower planar SiO2 substrate 92 having diffraction grating elements 94 regularly arranged thereon in the fashion described above with respect to FIGS. 1 and 3. The two substrates 88, 92 are separated by a liquid crystal layer 98 which is suitably contained and which acts as an optically transparent bearing between the two substrates 88, 92 just as the layer 82 acts in the embodiment of FIG. 6.
Between a frame 96 and the left side of the substrate 88 is a metal element having a known coefficient of thermal expansion which responds to temperature changes to cause a lateral shift in the position of the substrate 88 relative to the adjacent frame 96. Similarly, a second element 102 is mounted between the frame 96 and the right edge of the substrate 92 so as to cause a shift in the lateral shift in that substrate and the grating elements 94 in response to ambient temperature changes. The two shifts created by the metal elements 100, 102 are cumulative and, when properly calibrated to achieve a shift of one-quarter of the period of the gratings 90, 94 over the temperature range of interest, can produce a valving or switching function wherein the incident sunlight 104 is fully transmitted into the room 86 under low temperature conditions but is essentially fully blocked when ambient temperature becomes high. The embodiment of FIG. 7 can be used with additional sputter-deposited filters for selected wavelengths.
It is to be understood that the invention has been described with reference to specific materials and specific practical applications and that these descriptions are illustrative rather than limiting. For a definition of the invention reference should be taken to the accompanying claims.

## Claims

1. An optical device comprising:
first and second diffraction grating dividers each made up of a distributed plurality of grating elements having a predetermined, regular geometry and being formed of a first material immersed in a medium and disposed on a substrate wherein the geometries of said grating dividers are substantially identical;
the first material having a refractive index that is greater than the refractive index of the substrate, and the substrate having a refractive index which is greater than the refractive index of the medium;
said grating dividers being arranged in mutually inverted but spaced apart and substantially aligned relationship; and
means for varying the lateral alignment of the first and second grating dividers to vary the degree to which incident light is transmitted therethrough.

2. An optical device as defined in claim 1 wherein the medium is air.

3. An optical device as defined in claim 1 wherein the first and second grating dividers are spaced apart by a volume of liquid crystal.

4. An optical device as defined in claim 1 wherein the grating elements are made of TiO2.

5. An optical device as defined in claim 1 wherein the substrates are made of fused SiO2.

6. An optical device as defined in claim 1 wherein the grating dividers are defined by successive ridges and grooves having a periodicity p, a height h and a fill factor r wherein p is in the range of about 0.87 to 0.93 λ, r is in the range is about 0.22 to 0.32 and h is in the range of about 0.24 to 0.34 λ.

7. A light valve comprising:
first and second diffraction grating dividers each made up of a distributed plurality of grating elements having predetermined, regular geometries and formed of a first material immersed in a medium and disposed on a substrate of a second material having a critical angle of refraction wherein the geometries of said grating dividers and elements are substantially identical and produce a refraction angle of greater than the critical angle of the second material in said medium;
said gratings being arranged in mutually inverted relationship; one side of the first grating divider providing an input to incident light, the opposite another side of the second grating divider opposite said one side providing an output to transmitted light; and
the elements of first and second grating dividers being arranged to permit variations in lateral alignment which variations vary the transmission of light through said gratings.

8. A light valve as defined in claim 7 wherein the medium is air.

9. A light valve as defined in claim 7 wherein there is a volume of liquid crystal between the first and second substrates.

10. A light valve as defined in claim 7 wherein the grating elements are formed of TiO2.

11. A light valve as defined in claim 7 wherein the substrates are made of SiO2.

12. A light valve as defined in claim 7 wherein the means for varying comprises at least one MEMS.

13. A light valve as defined in claim 7 wherein the means comprises a thermal expansion device whereby the variation in lateral alignment is a function of temperature.

14. A variably light-transmissive window comprising:
first and second diffraction grating dividers each made up of a distributed plurality of grating elements having regular geometries and formed of a first material immersed in a medium and mounted on a substrate of a second material having a critical angle of refraction wherein the geometries of said grating elements are substantially identical and produce a refraction angle of greater than the critical angle of the second material in said medium;
said grating dividers being arranged in mutually inverted, substantially laterally aligned, spaced-apart relationship with an interface substance having a refraction index of about 1 therebetween;
in a first side of the first grating divider providing an input surface to incident light, a first side of the second grating divider providing an output surface to transmitted light; and
means for varying the lateral alignment between the elements of first and second grating dividers thereby to vary the transmission of visible light through said gratings.

15. A variably light-transmissive window as defined in claim 14 wherein the medium is air.

16. A variably light-transmissive window as defined in claim 14 wherein the substance is liquid crystal.

17. A variably light-transmissive window as defined in claim 14 wherein the grating elements are formed of TiO2.

18. A variable light-transmissive window as defined in claim 14 wherein the substrates are made of SiO2.

19. A variable light-transmissive window as defined in claim 14 wherein the means for varying comprises at least one MEMS.

20. A variable light-transmissive window as defined in claim 14 wherein the means comprises a thermal expansion device whereby the variation in lateral alignment is a function of temperature.

21. A light valve comprising:
first and second diffraction gratings comprising periodically arranged elements of a first refractive index on a substrate of a second refractive index less than the first refractive index; the elements of said gratings being of common periodicity;
a liquid medium;
said gratings being arranged in opposed and aligned relationship with said liquid medium therebetween; and
means for changing the alignment between said gratings thereby to vary the transmission of incident polarized light through the valve.